The proposed project is directed at determining the extent to which the responses of mammalian pigment cells to UV- and X-radiations arise from direct modification of the irradiated cell or are dependent on events initiated within the particular cellular community of which it is a part. The problem of pigment cell "niche" and radiosensitivity is examined and is subjected to experimental analysis. The significance of melanin pigment as a radioprotective agent is evaluated in the case of epidermal melanocytes. Particular emphasis is placed on determining the consequences of the proposal that the epidermal melanin unit, consisting of a melanocyte together with its constellation of associated epidermal cells (keratinocytes), is the fundamental integrated unit of pigmentary function in the epidermis of vertebrates. The functional and morphological properties of the epidermal melanin unit are examined in the skin of vertebrates representative of each of the major classes. Particular attention is given to the roles that genes and hormones play in modifying the responses of mammalian melanocytes to ionizing and ultraviolet radiations. The cellular basis for radiation-induced changes in melanocyte populations and the nature of cellular interactions during pigmentation of skin and hair are also examined.